Fix padded_length() function to work on 32bit systems.

On 32 bit systems item_size_type and size_t are the same size, so there was a
duplicate definition here. See also #56.
Fix floating point comparison in a test.
Hack to run one test only on 64bit machines.

Without this workaround, the test did not compile on 32bit machines.
Simplify threading code.

Use std::async instead of our own CheckedTask class. Added two utility
functions to make usage just as easy. This gets rid of some warnings from
the valgrind helgrind tool.
Start of an AppVeyor configuration.

Stolen from OSRM project and edited. No idea whether it will run...
Remove use of SharedPtrWrapper.

Introduced as a hack to make libosmium work on Windows, this should not be
necessary now that we have the function_wrapper.
Remove unused methods from thread queue.
Some tiny cleanups of function_wrapper class.